Non-structural carbohydrate metabolism coordinates temperature-mediated flowering regulation in Hippeastrum

Hippeastrum (Amaryllis) is a bulbous plant valued for its large, showy flowers and distinctive floral morphology. To achieve year-round flowering, Hippeastrum bulbs are typically subjected to low-temperature treatment. However, the physiological and molecular mechanisms underlying this process are still not well understood. In this study, we exposed Hippeastrum 'Alfresco' bulbs to low-temperature treatment (10 °C) for different durations (0, 15, 30, 45, and 60 days) before they were planted in a greenhouse. The results revealed that low-temperature treatment advanced flowering phenology; however, floral bud differentiation occurred before low-temperature treatment. Among all the treatments, 45 days of low-temperature exposure most effectively accelerated scape elongation and flowering and extended the ornamental period. Analysis of non-structural carbohydrates (NSCs) revealed that a 45-day low temperature induced starch breakdown and increased soluble sugars at cylindrical scales. Spatiotemporal analysis of NSCs in the basal plate, cylindrical scales and semicyclic scales of bulbs treated for 45 days revealed significant depletion of starch, sucrose, and fructose accompanied by glucose accumulation during scape elongation. Transcriptome analysis identified a significant enrichment of genes involved in starch and sucrose metabolism in the basal plate during the scape elongation, especially HpSUS1 and HpFRK1. qRT-PCR analysis, which is consistent with the enzyme activity results, confirmed increased expression levels of HpSUS1 and HpFRK1 during scape elongation, which coincides with sucrose depletion in the semicyclic scales adjacent to developing floral buds. To further validate the role of sugars, bulbs treated at 10 °C for 45 days were grown hydroponically and supplemented with sucrose and glucose, resulting in increased flower longevity. Collectively, these findings demonstrate that low temperature influences NSC metabolism through the coordinated activity of HpSUS1 and HpFRK1, which mediate temperature-dependent floral regulation in Hippeastrum. This study provides fundamental insights into ornamental crop physiology and practical strategies for optimizing floral traits.